The term "electrotransport" as used herein refers generally to the delivery of an agent (eg, a drug) through a membrane, such as skin, mucous membrane, or nails, which delivery is induced by application of an electrical potential. For example, a beneficial therapeutic agent may be introduced into the systemic circulation of a human body by electrotransport delivery through the skin. A widely used electrotransport process, iontophoresis, involves the electrically induced transport of charged ions. Another type of electrotransport, electroosmosis, involves the flow of a liquid, which liquid contains the agent to be delivered, under the influence of an electric field. Still another type of electrotransport process, electroporation, involves the formation of transiently-existing pores in a biological membrane by the application of an electric field, through which pores an agent can be delivered either passively (ie, without electrical assistance) or actively (ie, under the influence of an electric potential). However, in any given electrotransport process, more than one of these processes may be occurring simultaneously to a certain extent.
Accordingly, "electrotransport", as used herein, should be given its broadest possible interpretation so that it includes the electrically induced or enhanced transport of at least one agent, which may be charged, uncharged, or a mixture thereof, regardless of the specific mechanism or mechanisms by which the agent actually is transported.
Electrotransport devices generally use at least two electrodes which are in electrical contact with some portion of the skin, nails, mucous membrane, or other surface of the body. One electrode, commonly referred to as the "donor" or "active" electrode, is the electrode from which the agent is delivered into the body. The other electrode, typically termed the "counter" or "return" electrode, serves to close the electrical circuit through the body. For example, if the agent to be delivered is positively charged, ie a cation, then the anode will be the active or donor electrode, while the cathode serves to complete the circuit. Alternatively, if an agent is negatively charged, ie an anion, the cathode will be the donor electrode. Additionally, both the anode and cathode may be considered donor electrodes if both anionic and cationic agent ions are to be delivered.
Furthermore, electrotransport delivery systems generally require at least one reservoir or source of the agent to be delivered to the body. Examples of such donor reservoirs include a pouch or cavity, a porous sponge or pad, and a hydrophilic polymer or a gel matrix. Such donor reservoirs are electrically connected to, and positioned between, the anode or cathode and the body surface, to provide a fixed or renewable source of one or more agents or drugs. Electrotransport devices also have an electrical power source such as one or more batteries. Typically, one pole of the power source is connected to the donor electrode, while the opposite pole is connected to the counter electrode. In addition, some electrotransport devices have an electrical controller which controls the current applied through the electrodes, thereby regulating the rate of agent delivery. Furthermore, passive flux control membranes, adhesives for maintaining device contact with a body surface, insulating members, and impermeable backing members are some other potential components of electrotransport devices.
Although the advantages of electrotransport delivery are numerous (eg, enhanced transmembrane flux of beneficial agents compared to passive, ie, non-electrically assisted flux; precise control of agent delivery, including patterned delivery, etc.), there are disadvantages under certain application conditions. One potential problem with electrotransport transdermal delivery is skin irritation. For instance, applying an electric current through skin under certain conditions has been known to cause skin irritation. See for example, "Skin Biological Issues in Electrically Enhanced Transdermal Delivery", P. Ledger, Advanced Drug Delivery Reviews, Vol. 9 (1992), pp 289-307.
The prior art has recognized that the pH of the solution of the drug or agent being delivered (ie, the pH of the donor reservoir in an electrotransport device) can have an effect on skin irritation. According to "Structure-Transport Relationships in Transdermal Iontophoresis" by Yoshida et al, Ad. Drug Del. Rev, (1992), 9, 239-264, the preferred pH range of the donor reservoir, for avoiding skin irritation is 3 to 8. Outside this pH range, according to this reference, irritation and/or damage of the stratum corneum can occur. Furthermore, previous disclosures relating to minimizing skin irritation from electrotransport devices have concentrated on the active or donor reservoir. However, electrotransport devices apply as much current through the counter electrode as through the donor electrode, and hence, skin irritation, erythema and/or damage due solely to application of electric current also occurs beneath the counter reservoir or counter electrode. In a typical electrotransport device, the area of device/skin contact beneath the counter reservoir is nearly equivalent to the area beneath the donor reservoir. Hence, skin erythema, irritation, and/or damage in the counter reservoir contact area may be similar in magnitude to that in the donor reservoir contact area.